It is well known that modulation formats generated electrically can be translated to optical frequencies by use of an optical single sideband (SSB) modulator. Because of the increased interest in the generation of more complex modulation formats to improve spectral efficiency there is a need for efficient and cost-effective methods for optical SSB modulation.
The usual method of generating optical SSB is to use one or more Mach-Zehnder Interferometer (MZI) arrangements. The arms of the MZI contain optical phase adjusters and electro-optic phase modulators. These are driven by a combination of electrical signals. The correct phase relationships to cancel (null) out the carrier and one set of sidebands is achieved using an appropriate combination of optical path lengths and control of the phase of the electrical drive signals. The resultant modulator is large (several cm long), often requiring special travelling-wave electrode structures, and is not well suited to optical integration in a photonic integrated circuit (PIC).